Polymers are widely employed to produce functional and decorative articles. These articles are predominantly produced via a melt forming process, such as injection molding, extrusion molding, extrusion blow molding, thermoforming, or 3D printing. Pigments or dyes are often added to the polymers to provide desirable visual or functional effects, including but not limited to, opacity, light barrier, colors, or pearlescence or other special effects. These pigments or dyes are most frequently added to the polymers during the melt processing operation in the form of a masterbatch or concentrate, rather than added as individual ingredients to the polymer during polymer resin manufacture or melt-mix compounding.
There are two predominant methods to add pigments or dyes to polymers during melt processing. In the first method, a solid “masterbatch” color concentrate is produced, wherein the dry pigments or dyes are melt compounded into a polymer resin at a high concentration, and the masterbatch is then pelletized. The masterbatch pellets are subsequently blended with unpigmented “natural” resin pellets and perhaps other ingredients, and the mixture subjected to the molding process to produce the desired article. Typical pigment concentrations in a masterbatch range from 2-50%, and in the final article can range from 0.001 to 5%.
A second method is to add the pigments or dyes directly to the polymer during melt processing. In this method, the pigments are typically first dispersed in a liquid compatible with the polymer being processed, and the liquid dispersions are metered into the melt process at a predetermined rate in order to achieve the desired pigment level in the final article. The liquid employed is often referred to as a “vehicle” or “carrier.” Typical pigment concentrations in a liquid dispersion range from 30-70%, and in the final article can range from 0.001 to 5%. In general, adding pigments via liquid dispersions during melt processing is the more economical process.
Because of the difficulty of achieving good dispersions of dry pigments in a liquid or in a polymer matrix, dispersion aids are often employed. Generally, the function of the dispersion aids is to improve the compatibility/wettability of the dry hydrophilic pigment with the more hydrophobic polymer or liquid. The need for such compatibilization is greatest where the pigments are highly hydrophilic and the polymer and/or liquid are highly hydrophobic. Examples of such hydrophilic/hydrophobic combinations that benefit from compatibilization include, but are not limited to, titanium dioxide in polyethylene, iron oxides in polypropylene, and micas in polyethylene. When compatibilization is insufficient, clumping of pigment particles is common, which results in visual and functional imperfections such as speckling, streaking, inhomogeneous coloring, and the like.
While compatibilization of the pigment with the polymer matrix is desirable regardless of the method used to add the pigments, the need for compatibilization is more acute in the case of liquid dispersions. The increased requirement for compatibilization with liquid dispersions arises because there is generally less time in the polymer melt for mixing to occur, the melt processing conditions utilized to manufacture articles are less aggressive that those available for melt compounding of masterbatches, the liquid/polymer ratios employed are generally much lower than the masterbatch/polymer ratios, and there are limits to how much liquid can be added to the polymer melt without adversely affecting the polymer's processability or physical performance properties. A further constraint for liquid pigment dispersions is that the pigment dispersions need to be substantially stable to settling and yet must be fluid enough to be pumped through narrow-gauge tubing and other equipment. In addition, the compatibilization agent/pigment must be thermally and hydrolytically stable, so that loss of compatibilization does not occur in heating the liquid dispersion up to the polymer processing temperature, and compatibilization is not lost under conditions of extended storage.
For polymer, pigment, and processing conditions that are less demanding, state of the art compatibilization is sufficient, and liquid pigment dispersions are the preferred method of incorporating pigments into polymeric articles. Examples of such less demanding conditions include, for example, polyester (PET) injection molding with titanium dioxide pigments using ethoxylated sorbitan trioleate as a compatibilizer. Conversely, for more demanding applications, such as extrusion blow molding of polyolefins, the dominant method for incorporating pigments is via masterbatches.